Various specialized tires such as high performance tires, snow tires and studless tires are now available on the market, and are selected according to the particular need and desire of the user. In particular, studless tires are receiving a wide acceptance because they allow the vehicle to travel over snowy or icy road surfaces without losing traction, and over normal road surfaces without damaging the ride comfort of the vehicle.
However, those specialized tires are known to significantly change the handling of the vehicle, in particular in high speed ranges. Therefore, the vehicle operator is required to adapt himself to the different handling of the vehicle owing to the changing of the tires, and this may cause a significant stress on him. FIGS. 9(a) to 9(d) show graphs for demonstrating the handling of a conventional front wheel steering vehicle. According to these graphs, it can be seen that studless tires give rise to significantly reduced gains and large phase delays of the lateral response of the vehicle, such as yaw rate and lateral acceleration, near 1 Hz as compared to normal tires. In this application "normal tires" refers to radial tires commonly used on vehicles.
A number of proposals have been made to improve the steering stability of a vehicle by using a front and rear wheel steering system. Two different ways of controlling the steering angle of the rear wheels are known. The rear wheels may be steered according to the principle of feed-forward control or according to a predetermined pattern which is given as a mathematical function of the steering wheel angle, the angular speed of the steering wheel, the vehicle speed and other data. The rear wheels may also be steered according to the principle of feedback control or according to the response data of the vehicle such as lateral acceleration and yaw rate. However, these control principles are designed for predetermined control parameters, and produce optimum results only under standard conditions. When the tires are changed to specialized tires such as studless tires, the vehicle demonstrates a different lateral response; the gain diminishes, and the phase delay increases in terms of yaw rate or lateral acceleration for a given steering input. Such deviations from standard conditions could prevent the rear from being steered in a satisfactory fashion.